Field of the Invention
The present invention relates to a capped oil soluble polyalkylene glycol having a kinematic viscosity at 100° C. below 4.5 centiStokes and a viscosity index greater than 150.
Introduction
A macro-trend across the lubricant industry is the search for and the development of base fluids that offer lower viscosities than current base oils but preferably with higher viscosity index values. Formulations that contain lower viscosity base oils can offer lower friction losses inside equipment and therefore improved energy efficiency.
Today most lubricant compositions use hydrocarbon oils as the base oil. However it is well known that as the viscosity is lowered for hydrocarbon oils, the viscosity index is also usually lowered. It is desirable to have a base oil with a kinematic viscosity at 100° C. that is less than 4.5 centiStokes (cSt), preferably four cSt or less, and a viscosity index greater than 150. Today API (American Petroleum Institute) Group I and II hydrocarbon oils that have a kinematic viscosity at 100° C. that is 4.5 cSt or less and have viscosity index values in the range 80-120 or lower. Synthetic hydrocarbons such as polyalphaolefins have values which are slightly higher. For example Synfluid PAO-4 from Chevron Phillips has a kinematic viscosity at 100° C. that is four cSt and a viscosity index value that is 124.
Traditional polyalkylene glycols (PAGs) such as co-polymers of ethylene oxide (EO) and propylene oxide (PO) and homo-polymers of propylene oxide offer higher viscosity index values than hydrocarbon oils for similar viscosity grades. However, the viscosity index still decreases with viscosity for the PAGs. Capping these polymers (EO and/or PO types) with an alkyl or aryl group can increase their viscosity index values by about 20-50 units. For example PAGs having a kinematic viscosity at 100° C. of 5-20 cSt in which methyl capping results in viscosity index increase of 20-50 over the non-capped PAG to yield values of 180-250. Capped PAGs of ethylene oxide and propylene oxide have a drawback of being expensive components of lubricant formulations. Another major disadvantage is that they are not soluble in hydrocarbon oils at higher treat levels (for example, greater than 10 percent by weight). Therefore they cannot be used as co-base oils in hydrocarbon oils.
In order to overcome hydrocarbon solubility challenges, a new range of polyalkylene glycols called oil soluble polyalkylene glycols (OSP) were developed. OSPs contain 1,2-butylene oxide as a building block. There is now a commercially available family of OSPs that are dodecanol initiated PO/BO (50/50 weight ratio) copolymers. One end of the polymer is terminated with a dodecanol group but the other has a free hydroxyl group. Their commercial names are UCON™ OSP-18, 32, 46, 68, 150 and 220 (UCON is a trademark of Union Carbide Corporation). However low viscosity OSPs have quite low viscosity index values. For example UCON OSP-18 has a kinematic viscosity at 100° C. of four cSt and a viscosity index value of 123. It is desirable to have a kinematic viscosity at 100° C. of 4.5 or less, preferably four cSt or less, and a viscosity index of 150.
Overall, there is a desire to achieve low kinematic viscosities (less than 4.5 cSt at 100° C.) with high (above 150) viscosity index values in an oil soluble polyalkylene glycol.